1. Field of the Invention
This invention relates generally to the cell structure, device configuration and fabrication process of power semiconductor devices. More particularly, this invention relates to a novel and improved device configuration and processes to manufacture trench MOSFET device with solderable front metal.
2. Description of the Related Art
Conventional configurations of the trench semiconductor power devices in providing front metal as source metal contact and gate pad are confronted with the technical difficulties that the device configurations have limited capabilities of thermal conduction. The thermal conduction issues become even more pronounced when the semiconductor power devices are manufactured with reduced die size.
Referring to FIGS. 1A and 1B for a side cross sectional view and top view of a device configuration as disclosed in U.S. Pat. Nos. 6,462,376 and 6,888,196 where the trench MOSFET devices have two dimensional source contact implemented with the tungsten plugs that extend into the source/body regions. The source metal contact is then electrically connected to the front metal composed of aluminum alloys. Aluminum wires are then formed on top of the front metal-layer composed of aluminum alloys. The thermal conduction of such device is limited when the die size is reduced due to an increase in the cell density. The thermal conduction path in a device shown in FIGS. 1A and 1B is through the back-metal (not shown) formed on N+ substrate with the heat conducted through the silicon, the tungsten plug, the aluminum front metal, and the aluminum wire to lead frame. The reduction in device die size results in less silicon area to transfer the heat generated during the times when the device is turned on. Moreover, the conventional aluminum wire bonding cannot provide adequate contact area to the front metal to efficiently transfer the heat to the lead frame. For these reasons, conventional device configurations of the semiconductor power devices encounter a technical limitation that would limit the device performances due the heat conduction difficulties, especially when the cell density is increased with the decreased die size.
As more semiconductor power devices are employed in portable devices, the die size for manufacturing the semiconductor power devices is further reduced and the cell density increased. Therefore, an urgent demand now exists in the art of designing and manufacturing semiconductor power devices to provide new and improved device configuration to resolve such limitations.